Computed tomography (CT) and laminography systems are able to image three dimensional (3D) structures of samples, particularly their embedded structures and often without any destructive or invasive process. These techniques have found a wide range of applications from non-destructive integrated circuit testing to medical/biological imaging. In these systems, a series of projection images are acquired at different view angles to gain different perspectives of the specimen. These projections are then mathematically assembled to reconstruct a 3D image that represents the 3D structure of the specimen. X-ray radiation is typically used in these applications as it provides a good combination of high penetration, low scattering, and relatively simple absorption characteristics.
A key requirement in producing the 3D images in both CT and laminography systems is that all projections must be aligned so that the sample appears to rotate through a common rotation axis. In most systems with imaging resolutions coarser than tens of micrometers, the mechanical rotation stage accuracy is sufficient to maintain apparent rotation axis in projection images without any additional data processing. In these cases, a simple pre-calibration is need to determine the systematic shift error at each view angle and true rotation center using a well-known test sample. However, with higher resolution imaging systems such as micro-CT or nano-CT systems, the asynchronous error of most mechanical stages will exceed the resolution, and a simple calibration will not be sufficient to maintain the alignment accuracy. In these cases, an additional alignment step is typically needed to remove the shift errors.
Several approaches have been used to mitigate mechanical stage alignment errors. Two commonly used techniques are:
(1) Using intrinsic features in the sample or introducing artificial alignment targets into the sample and align each projection to this target. This approach generally requires some distinct sharp/high-resolution features inside the sample that can be imaged at high-resolution at all view angles. Otherwise, some modification will be required to introduce artificial features into the sample. The later approach is often used in CT with transmission electron microscopes (TEM) when colloidal gold particles are introduced into samples as alignment targets.
(2) Fabricating alignment target on the sample holder and use an integrated metrology system to monitor the position of the sample holder. The monitoring system will produce shift data for each view angle that will be used in the alignment routine. Typical monitoring systems can be based on various technologies such as mechanical micrometers, capacitance sensors, encoders, and laser interferometers, for example.